Research progress of slit coating technology

0 Introduction

Slit coating is a coating technique in which the coating liquid is pressed out along the mold gap and transferred to the moving substrate under fixed pressure. In 1954, Bequin and others of Kodak invented Slot Die Coating. Slot Die Coating (SDC) has many advantages, such as: high coating speed, good consistency of film thickness, wide viscosity range of coating solution, few coating defects (closed-loop system), high utilization rate of coating solution and can be applied in three layers at the same time; Its shortcomings: high equipment cost, high technical knowledge requirements for operators, high installation and operation requirements, high precision coating head high maintenance costs. SDC is used in a wide range of fields, from traditional film and paper to solar cells and lithium-ion batteries. For example :OLED conductive film, optical film of liquid crystal panel, solar cell backplane, lithium ion secondary battery and so on. Although SDC technology is a relatively mature technology, for industrial production, small technical improvements may play a major role in cost reduction and produce huge economic benefits. Therefore, the research of SDC technology is of great significance. At present, the research of SDC technology mainly focuses on equipment performance optimization, coating efficiency improvement, model research and so on.

In this paper, the research progress of SDC at home and abroad is reviewed from SDC typical equipment, process, coating head structure, model and simulation, and common defect mechanism analysis, and the future development direction of SDC technology is prospected.

1 Typical SDC device

SDC equipment is mainly composed of rewinding system, coating system and drying system. The following is a brief introduction to the coating system.

The coating system, as shown in Figure 1(a), mainly includes a feeding unit and a coating unit, and the feeding unit includes a storage tank, a conveying pump, a filtration device, etc. The coating mechanism is mainly composed of a valve system that controls the coating gap, a pressure control system and a coating head. The coating head consists of three parts [Figure 1(b)] : the upper Die (Up Die), the lower die (Down Die), and a thin gasket (Shim) installed between the upper die and the lower die. During the coating process, under the action of pressure, the coating liquid is extruded from the gap between the upper and lower die, forming liquid beads between the moving substrate and transferring to the surface of the substrate to form a wet film; The formation of liquid beads is the key to film formation. The key parameters of liquid beads include the formation and position of the upper and lower meniscus, and the position of static contact line and dynamic contact line, as shown in Figure 1 (c). Lateral clearance is mainly controlled by the valve, while clearance in the MD(Machine Direction) direction is controlled by the Shim, as shown in Figure 1(d).

Quanzhou Dajiang coating equipment Co., LTD

Figure 1 Schematic diagram of typical SDC devices

Fig.1 Typical schematic diagram for SDC

Because SDC has high requirements on the mold accuracy of the coating head, most of the coating head molds used in SDC equipment are mainly imported. The main mold suppliers are Mitsubishi, EDI, Allies Die, etc. According to the adjustment mode of coating head, it is divided into fixed type and adjustable type. The stationary mold adjusts the uniformity of coating weight by adjusting the coating gap (gap between the lip and the back roll); The adjustable type adjusts the weight uniformity by adjusting the gap between the upper and lower lip. Stripe coating is generally achieved by the shape of the gasket. Mitsubishi designed a gasketless mold, cutting grooves directly on the lip, eliminating the gasket and facilitating personnel operation; The disadvantage is that different stripe specifications require different molds, and the cost is high. Suitable for the production of a single variety of products.

2 SDC process research

SDC process research can be divided into two levels, one is engineering technology, the other is scientific research. From the perspective of engineering technology, the research of SDC mainly includes the optimization of equipment structure, the determination of coating window, process control and so on. From the perspective of scientific research, the study of SDC mainly involves the law of fluid motion, the formation mechanism of liquid film, the establishment of coating model and the process simulation. The following will review the recent research results of SDC from three aspects: process influencing factors, equipment structure, model and simulation.

2.1 Process influencing factors

2.1.1 The startup process from startup to steady state coating is very critical to the smooth progress of the entire coating.

Shikhmurzaev et al and Nagai et al studied the expansion process of liquid beads on substrate during start-up. Hirt et al. simulated the startup process of SDC. The results of Yang et al. show that rapidly increasing liquid flow rate can reduce the edge effect during gap coating when steady state coating is achieved. Yi-Rong Chang et al. studied the formation process of liquid beads during SDC startup by real-time observation and simulation with a microscope, and found that there were four different modes in the formation of steady liquid beads. It was found that pre-wetting of the substrate, increasing the viscosity of the coating liquid, increasing the coating gap and slit gap, and shortening the lip length could effectively shorten the startup time.

2.1.2 Coating Gap fluctuation in the SDC process, the coating gap fluctuation often occurs due to mechanical reasons, which has a great impact on the uniformity of coating thickness. Eduardo et al. studied the relationship between the periodic vibration of coating gap and the fluctuation of film thickness through computer simulation, and found that the amplification factor (αh=hm/Hm, hm is the amplitude of film thickness, Hm is the perturbation amplitude of the coating gap) and wave number (k=2πf/Vw, where Vw is the substrate velocity at the time of coating) have a good matching relationship: when K < 1, there is an adjustment range of the coating gap, and with the increase of K value, this range moves towards the low value. If the coating gap is at a fixed value, The fluctuation of coating film thickness can be suppressed by the fine adjustment of vacuum pressure. When K > 1, when the coating gap is fixed, the value of the vacuum pressure can be adjusted in a lower value range.

2.1.3 Process window The coating window is the range within which process parameters can be adjusted under the premise of acceptable film quality, beyond which defects will occur. Therefore, determining the coating window is a key step to ensure the quality of the coating film. Schmitt et al. studied the coating window of lithium ion battery anode paste, analyzed the process conditions of coating defects and the influence of particles on coating. Hyunkyoo et al. studied the correlation between operating conditions, film thickness and fringe width by means of experiment and statistical analysis. The substrate moving speed is a significant influence factor on film thickness and fringe width, and the flow ratio of feed and coating gap have no influence on fringe width, but will affect film thickness. Yu-Rong Chang et al. studied the thickness of small wet film in the SDC process. The critical Reynolds coefficient of the coating area is given. The position of the meniscus downstream of the coating determines the type of coating area, while the position of the meniscus upstream determines the type of coating defect. Feng Liu et al. introduced SDC into the production of organic photovoltaic cells (OPV) and combined it with grazing-incidence small Angle X-ray diffraction (GISAXS) to study the surface morphology changes of coating films under different drying conditions.

In addition to the influence of fluid variation, gap fluctuation and coating window, the geometry of coating head and installation Angle also have significant influence on coating.

2.2 Influence of coating head structure on coating

Based on the SDC of protruding Shim, Soonil Hong et al. studied the correlation between the surface morphology of the film layer and the protruding length of Shim, the substrate temperature and the coating speed, and gave the equation for determining the thickness of protruding Shim coating. Danmer et al. studied the influence of lip design of different structures on SDC through models, analyzed the influence of different structure design on steady-state solution and the adjustment range of process operation parameters. Oldrich et al. studied the influence of the R-angle curvature radius of the lip on the stability of the static contact line and fluid stability, and determined the small curvature radius of the stable static contact line. Danmer et al. analyzed the coating process of double-layer SDC through Die structure changes, coating speed, vacuum pressure, flow rate, etc. The results showed that external periodic disturbance factors, such as coating head and flow pump, should be considered when designing Die structure to avoid resonance frequency. In addition, the influence of periodic external disturbance on coating can be effectively avoided through Die structure design optimization. Romero et al. also studied the influence of Die structure on coating through simulation. Ahn et al studied the SDC process based on the Viscocapillary Model and frequency response analysis based on different upper lip structures and different flow rates. With different upper lip structures, changes in the position of the meniscus of the upstream liquid bead significantly affect the coating dynamic behavior. It can reduce the sensitivity of coating system to external periodic disturbance and improve the robustness of the process. Jaewook et al. also studied the influence of Die geometry on Tensioned (web-over-slot Die) SDC coating through computer simulation, and the simulation results showed that the coating window could be expanded through appropriate Die structure design.

2.3 Influence of mounting Angle of coating head on coating

The installation Angle of the Die is generally at 6 o 'clock or 12 o 'clock, and it is now more common to install it at 4 o 'clock or 5 o 'clock, mainly for the convenience of operation and to prevent the discharge of the coating liquid when the feed pump is closed. On the other hand, the installation Angle of the Die should also consider the profile of the lip, the size of the coating gap, the hardness of the back roll, the diameter of the back roll, the dislocation size of the lip, the nature of the coating liquid, the pressure between the Die head and the back roll and other factors. Chang et al. made a comparative study on the influence of the installation Angle of the coating head on the SDC process, and found that compared with the horizontal installation, the coating speed of the vertical installation was much higher than that of the horizontal installation. When installed horizontally and vertically, the main difference between the coating is the difference in the length of the formed liquid beads. There is a correlation between the wet film thickness and the Reynolds coefficient. According to the Reynolds coefficient, the wet film thickness is mapped with the capillary number, and it is found that there are three different wet film thickness regions. In the first region, the wet film thickness gradually increases, gradually flattens to the second region, and sharply decreases to the third region. When horizontal installation, there are one and two zones, and when vertical installation, there are two and three zones.